Hydrogen is a very promising environmentally friendly fuel. The combustion of hydrogen in electrical power generating devices (PGDs), such as fuel cells, directly produces electricity in a pollution-free way with an efficiency that is much higher than that of heat engines, with water being the only byproduct. The high energy transformation efficiency of fuel cells may decrease significantly carbon dioxide emission, even when fossil fuels are used as a source of hydrogen. Conventionally, hydrogen production is carried out by the industrial-scale steam-reforming of natural gas, which is a multistage process that includes several reaction steps with heat-exchange between them, as steam-reforming is highly endothermic, followed by hydrogen extraction and purification. Water electrolysis is another well-known hydrogen generation process, which is, however, the most energy-demanding way to produce hydrogen unless renewable energy sources are used.
Unless produced on-site, hydrogen has to be compressed or liquefied, stored, transported and distributed to the end user. This, as well as the complexity and high energy demands of the conventional hydrogen generation processes, reduces significantly the overall efficiency of the hydrogen-based energy pathway, making the price of hydrogen uneconomical for practical applications. Therefore, on-site hydrogen generation from natural gas via an autonomous membrane reformer may be a very promising approach that can save costs of hydrogen storage and distribution (see FIG. 1). In addition, whereas conventional steam-reforming is a multistage process that includes several reaction steps including hydrogen separation, the use of an autothermal membrane reformer may help to combine all the processes in one compact and thermally independent unit (see FIG. 2).
Hydrogen production in membrane reactors via methane steam-reforming (MSR) has been extensively investigated in the last two decades. Methane steam-reforming is commonly described by three reactions: methane steam-reforming, i.e. converting methane to water and carbon monoxide (Eq.1), water gas shift (WGS, Eq.2), and the overall MSR reaction (Eq.3):CH4+H2O═CO+3H2 ΔH1=206 kJ/mol  (1)CO+H2O═CO2═H2 ΔH2=−41 kJ/mol  (2)CH4+2H2O═CO2+4H2 ΔH3=165 kJ/mol  (3)
The overall process is highly endothermic, and the heat required can be advantageously supplied via methane oxidation (MOx) (Eq.4):CH4+2O2=CO2+2H2O ΔH4=−803 kJ/mol  (4)
Methane steam-reforming is of particular interest due to the large natural gas resources around the world, and the fact that steam (i.e. water) may be easily supplied.
As it can be appreciated from the equations above, methane steam-reforming is a highly endothermic process (Xu J et al. AIChE Journal. 1989; 35:88-103). Nowadays, there are two conventional approaches for increasing the steam-reforming efficiency: autothermal steam-reforming and partial oxidation. Yet, the implementation of these processes has been often problematic due to several issues such as the selection of an adequate catalyst and the limitation in hydrogen separation capabilities. In most cases, wherein an exothermic reaction is directly coupled to the steam-reforming reaction, the catalyst should be suitable both for reforming and oxidation, and should be able to withstand hot spots emerging in the catalyst bed during methane oxidation. Regarding hydrogen separation, the advantages of combining steam-reforming and hydrogen separation in a single device have been described previously (Chen Y. et al., Appl Catal B Environ. 2008; 80:283-294; Patil C. S. et al., Chem Eng Sci. 2007; 62:2989-3007; Gallucci F. et al., Top Catal. 2008; 51:133-145; Chen Z. et al., Int J Hydrogen Energy. 2007; 32:2359-2366). However, when autothermal reforming or partial oxidation is employed in such membrane-assisted reforming applications, they may suffer from limited separation capability. When using air as a source of oxygen (using pure oxygen is neither a convenient nor cost effective approach), the reaction mixture is diluted by nitrogen, which causes a decrease in the hydrogen partial pressure difference (existing between the catalytic bed and the membrane interior) that drives hydrogen separation. Therefore, inadequate catalyst selection and issues in hydrogen separation may reduce significantly the membrane reformer efficiency, limit the hydrogen output, and induce low power densities.
Hydrogen production via membrane reformers has been described for instance in U.S. Pat. No. 5,861,137, US 2002/034458, US 2003/068260, US 2005/0178063, WO 2007/031713, US 2009/0170967, and US 2008/0019902.
However, prior art membrane reformers are generally not optimized in terms of overall energy conversion efficiency (from natural gas to hydrogen) and power density (hydrogen output per unit reformer volume), and are generally cumbersome. In particular, reformers which are not autonomous and lack an optimized thermal management, lead to low energy conversion efficiencies. Low power density usually results from poor technical integrations (e.g. multiple units) and from the implementation of bulky reactor designs (e.g. fluidized bed, thermal combustion).
Therefore, it is an object of the present invention to provide an energy efficient process for generating hydrogen.
It is a further object of the invention to provide an autothermal and compact apparatus for the production of hydrogen from a natural gas.
It is yet a further object of the invention to provide a system suitable to generate hydrogen from a natural gas and generate an electrical power suitable to be used by electrically driven components, machine, apparatus, and vehicles.
It is yet a further object of the invention to provide a method for manufacturing and determining the structural and functional characteristics of an autothermal heat exchanger packed-bed membrane reformer' (APBMR), able to fit a specific power generating device (PGD) having a determined power efficiency and a determined power output.
These and other objects of the invention will become apparent as the description proceeds.